The present invention generally relates to thermal interfaces across which heat is transferred. More particularly, this invention relates to the enhancement of heat transfer across a thermal interface through the inclusion of carbon nanotubes.
Reduction of interfacial thermal resistance remains a major challenge in the thermal management of many heat-generating engineered components and systems, including the chip-package interface of semiconductor devices, for example, a power integrated circuit (IC) and a heat sink or substrate. Thermal contact resistance exists at interfaces because of imperfections caused by microscopic asperities between contacting surfaces. Thermal interface materials (TIM's), such as solders, thermal greases, oils, gels and pastes, phase change materials (PCM's), and sheet-type solid materials, have been developed to reduce contact resistance. TIM's are generally more compliant than the contacting surfaces themselves, and tend to displace air gaps under the application of pressure to increase the extent of contact, thereby increasing thermal contact conductance. Some PCM's have achieved interfacial resistances of approximately 5 mm2K/W.
With flowable thermal interface materials, high thermal interface conductance is often achieved through enhanced spreadability and elasticity to fill the microscopic gaps between the interface surfaces. As a result of thermal fluctuations, during which the viscosity of a flowable TIM decreases with increasing temperature, nonuniform thermal expansion and deformation of the interface surfaces under a given thermal load can cause “pumping out” of the TIM from the outer edges of the interface. This phenomenon eventually causes part of the interface to dry out and results in dramatically increased thermal contact resistance.
In addition to those noted above, carbon fiber-based thermal interface composites have also been proposed, notable examples of which include TIM's containing carbon nanotubes (CNT's). As known in the art, CNT's are in the form of cylindrical carbon molecules with diameters less than one micrometer, and can be formed by various processes including chemical vapor deposition (CVD) on fine particles of a transition metal, particularly nickel or iron, that serve as a catalyst. Theory and experiments indicate that individual single-wall carbon nanotubes (SWNT's) and multi-wall carbon nanotubes (MWMT) exhibit extremely high phonon-dominated thermal conductivities of up to about 5000 to 8000 W/mK and 3000 W/mK at room temperature, respectively. The extremely high thermal conductivity of carbon nanotubes suggests many applications in various engineering fields, including electronics packaging. Prior research has indicated that the effective thermal conductivity of an oil containing about one volume percent CNT's is about 2.5 times the value of the base oil. Other research has indicated that an epoxy loaded with about one weight percent unpurified SWNT's exhibits an approximately 70% increase in thermal conductivity at 40K and an approximately 125% increase at room temperature. High thermal conductivity composite materials containing aligned carbon nanotubes for heat-spreading devices (microchannel heat sinks or heat pipes, etc.), have also been proposed.
In view of the above, the use of ultra-high thermal conductivity carbon nanotubes could potentially reduce interfacial resistance by an order of magnitude or more to satisfy the increasing power dissipation challenge. One approach to the use of CNT materials is to deposit CNT's as a coating for a thermal interface surface. For example, U.S. Patent Application Publication Nos. 2003/0231471 and 2004/0184241 each disclose an integrated circuit package having a TIM layer of diamond and a CNT array deposited by a plasma discharge process on a surface of the TIM layer. Another approach involves the use of CNT particles as a filler material to promote the conductivity of a soft TIM, such as a silicone or other polymeric material. Still another approach disclosed in EP1329953 is to deposit CNT bundles on a thermal interface surface, and then apply a soft TIM to the CNT layer. With this approach, the CNT bundles do not appear to form a continuous layer, but instead are spaced apart so that the soft TIM is present between adjacent CNT bundles.
Not withstanding the above-noted advancements, there is an ongoing desire to further improve thermal interface conduction and materials. In particular, as single-chip devices approach and exceed 1 W/mm2, improved thermal management strategies are required to achieve reliable packages for these devices.